Related Field
The present invention relates to a process and a machine for making a tissue paper web.
Description of Related Art
During manufacturing of tissue paper, a newly formed wet tissue paper web is taken from the forming section to a drying cylinder which may be a through-air-drying cylinder or a Yankee drying cylinder. If a Yankee drying cylinder is used, the tissue paper web is creped away from the surface of the Yankee drying cylinder when the web has been dried. When the tissue paper web is transferred to a Yankee drying cylinder, this is typically made in a press nip through which a felt is passed and where the press nip is formed between the Yankee drying cylinder and a press roll inside the loop of the felt. Such an arrangement is disclosed in, for example, U.S. Pat. No. 4,139,410. It has also been suggested that a nip against a drying cylinder can be an elongated nip where a shoe press unit is placed inside the loop of the felt. Such a solution is disclosed in for example U.S. Pat. No. 6,235,160 which shows an arrangement in which the felt and the paper web pass a suction roll located before the nip formed against a heated drying cylinder and where a shoe press unit inside the felt loop forms a nip against the heated drying cylinder. It is stated that the use of an elongated press nip (i.e. an extended nip) enables an intensive and volume-preserving drainage. In U.S. Pat. No. 6,780,282, an arrangement is disclosed which is largely similar to the arrangement of U.S. Pat. No. 6,235,160 but in which a hood is placed opposite a suctioned unit placed before the nip against the heated drying cylinder. The hood is said to comprise an overpressure fluid comprising at least one of overheated steam and dry and/or moist air. A similar arrangement is disclosed also in U.S. Pat. No. 6,083,349. In U.S. Pat. No. 6,083,349, hot air is blown against a shoe press that acts against a drying cylinder. Such an arrangement entails the disadvantage that the equipment may get dirty faster since other equipment such as a Yankee dryer coating shower may be used nearby. The web should preferably be heated at some distance away from the press nip against the drying cylinder. European patent No. 1959053 B1 discloses an arrangement that comprises a drying cylinder such as a Yankee cylinder and where a press element forms a press nip with the drying cylinder. In that document, it is described how a structured permeable fabric carries the paper web to the drying cylinder and it is stated that the structured permeable fabric may be a wire. Before the paper web has reached the Yankee cylinder, the paper web passes a through-flow drying apparatus having a feed air chamber and a waste air chamber formed by the suction box of a suction roll. Air fed to the feed air chamber is taken to some extent from a hood which is assigned to a drying cylinder such as a Yankee drying cylinder. The feed air chamber is placed inside the loop of the structured permeable fabric and air from the feed air chamber must pass through the structured permeable fabric before it reaches the paper web. A further permeable press fabric 24 is also arranged in a loop around the feed air chamber and is used to press the structured permeable fabric and the paper web against the suction roll. An additional dewatering fabric may also be arranged around the suction roll. The arrangement according to EP 1959053 is intended to cause drying of the paper web by means of hot air. Air which has left the suction roll may be recirculated back to the waste air chamber and a water separator is placed in the recirculation loop.
The object of the present invention is to provide an improved process and an improved machine for making a tissue paper web in which dewatering of the tissue paper web can be achieved in a more energy-efficient way.